Field diode detection of excess light conditions for spatial light modulator

ABSTRACT

An improved SLM that is capable of detecting when light incident on the SLM exceeds a predetermined threshold. A diode is fabricated around, or within the pixel array. Light incident on the array (and the diode) results in a current increase through the diode, which may detected and used to initiate a disable signal to control circuitry of the SLM.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates to spatial light modulators, and more particularly to detection of conditions in which the light incident on the spatial light modulator exceeds a predetermined intensity.

BACKGROUND OF THE INVENTION

[0002] A Digital Micromirror Device™ (DMD™ ) is a type of microelectromechanical systems (MEMS) device. Invented in 1987 at Texas Instruments Incorporated, the DMD is a fast, reflective digital spatial light modulator. It can be combined with image processing, memory, a light source, and optics to form a digital light processing system capable of projecting large, bright, high-contrast color images.

[0003] The DMD is fabricated using CMOS-like processes over a CMOS memory. It has an array of individually addressable mirror elements, each having an aluminum mirror that can reflect light in one of two directions depending on the state of an underlying memory cell. With the memory cell in a first state, the mirror rotates to +10 degrees. With the memory cell in a second state, the mirror rotates to −10 degrees. By combining the DMD with a suitable light source and projection optics, the mirror reflects incident light either into or out of the pupil of the projection lens. Thus, the first state of the mirror appears bright and the second state of the mirror appears dark. Gray scale is achieved by binary pulsewidth modulation of the incident light. Color is achieved by using color filters, either stationary or rotating, in combination with one, two, or three DMD chips.

SUMMARY OF THE INVENTION

[0004] One aspect of the invention is an improved spatial light modulator (SLM) having an array of pixel elements and control circuitry. In one embodiment, the improvement comprises a diode placed around at least a portion of the perimeter of the pixel array, the diode operable to conduct current in response to light incident on the pixel array. A disable circuit is operable to receive the current from the diode and to deliver a disable signal to the control circuitry when the current exceeds a predetermined amplitude.

[0005] An advantage of the invention is that it provides a cost effective means to detect overlight conditions, that is, conditions when light incident on the SLM exceeds a predetermined threshold. The collection diode may be fabricated using CMOS processes consistent with fabrication of the SLM. Existing voltages that are used to operate the SLM may be used to bias the diode.

[0006] The diode may be used to detect the overlight conditions in a manner that is transparent to the user of the SLM. It may be fabricated in a manner such that it is integral to the SLM and does not affect its operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a top plan view of an SLM having a light collection diode surrounding its pixel array.

[0008]FIG. 2 is a cross sectional view of the diode of FIG. 1.

[0009]FIG. 3 illustrates a disable circuit, which senses current through the diode and provides a disable signal to control circuitry of the SLM.

[0010]FIG. 4 illustrates an alternative embodiment, where the SLM array includes light collection diodes.

[0011]FIG. 5 is a cross sectional view of a diode of the SLM array of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The following discussion is related to use of a reverse-biased N-well diode as a light collector for a spatial light modulator (SLM). The didode forms a recombination region in the substrate of the SLM for photogenerated holes, which can be sensed as an increase in current through the diode. The sensed current may then be used to detect an “overlight” condition, that is, a condition in which the intensity of light incident on the SLM exceeds a predetermined intensity.

[0013] The diode may be fabricated using standard CMOS fabrication processes. The biasing voltages for the diode may be the same as those used for operation of the DMD. Further, the bias voltage may be from a low current power supply so that the signal to noise ratio of the photogenerated current is substantial.

[0014]FIG. 1 is a top view of an SLM 10 having a field diode 10 surrounding the pixel array 11. FIG. 2 is a cross sectional view of the diode 11 of FIG. 1.

[0015] For purposes of example herein SLM 10 is a DMD type SLM. As discussed in the Background, a DMD 10 is comprised of an array 11 of hundreds or thousands of micro-mirror elements, also referred to as “pixel elements”. Peripheral (active) circuitry 12 is used to load data to the pixel elements and control their operation. A control circuit 14 delivers control signals to the peripheral circuitry 12.

[0016] As illustrated in FIG. 2, the SLM substrate 21 is a p+ type material, in which the diode 13 is formed. A p− type epitaxial layer 22 is above the substrate 21. An n− well is fabricated in layer 22, and surrounds a smaller n+ region 23 at the surface of the DMD 10. A voltage, V_(diode), provides a reverse bias to diode 13. As indicated in FIG. 2, when light is incident on the surface of the SLM 10, photogenerated electrons recombine in the field diode 13 or at another positive bias junction.

[0017]FIG. 3 illustrates a disable circuit 13, used to disable the SLM 10 when the incident light exceeds a predetermined intensity. As stated above, light incident on the SLM 10 is sensed as an increase in the diode current. A sense amp 31 a senses this current and delivers it to a comparator 31 b. A comparator 31 b compares the diode current with a predetermined reference current. If the diode current exceeds the reference current, comparator 31 b delivers a signal to control circuit 14, which then disables the SLM 10.

[0018] When SLM 10 is a DMD, one possible DMD operating voltage that may be used for V_(diode) is the shield bias, which has no direct current. Another available voltage is the Vcc2 voltage.

[0019]FIG. 4 illustrates an alternative embodiment of the invention. Rather than surrounding the SLM array with a field diode, SLM 40 has a modified array 41, which includes positive biased n−well junctions.

[0020]FIG. 5 is a cross sectional view of an n−well junction 51 in array 41. An n− well 53 is formed in substrate 51 as described above in connection with FIG. 2. In a DMD type SLM 40, the existing array n−well may be used to form the diode. Two smaller doped regions, one p+ type 52 and one n+ type 53, are located within the n− well 53 at the surface of the SLM 40. As illustrated, when light is incident on the SLM, photogenerated electrons recombine in the array 41 to set up a signal current that will be superimposed on the existing current.

[0021] In both of the above-described embodiments, it may be desirable for the diode 13 or the diodes in array 41 to cover much of the exposure field of the SLM 10. The diode(s) may thereby rely on a large surface area for collection efficiency. This helps ensure that the collection current is immune to surface variations in the doping profiles of the silicon. Thus, in the example of FIG. 1, diode 13 completely surrounds the array 11. In the example of FIG. 4, the entire array 41 includes diodes. However, in other embodiments, diode 11 might surround only a portion of array 11, or only a portion of array 41 might include diodes.

[0022] Although the foregoing description is in terms of a DMD, the same concepts are applicable to other types of SLMs. For example, the embodiments of FIGS. 1 and 4 could be implemented to provide a disable signal for a liquid crystal device (LCD) array. It is anticipated that, consistent with FIGS. 1 and 4, the SLM will be comprised of an array such as arrays 11 and 41 and some sort of control circuitry 14 to be disabled.

[0023] Other Embodiments

[0024] Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. An improved spatial light modulator (SLM) having an array of pixel elements and control circuitry, the improvement comprising: a diode placed around at least a portion of the perimeter of the pixel array, the diode operable to conduct current in response to light incident on the pixel array; and a disable circuit operable to receive the current from the diode and to deliver a disable signal to the control circuitry when the current exceeds a predetermined amplitude.
 2. The improved SLM of claim 1, wherein the SLM is a DMD and the pixel array is an array of micro-mirror elements.
 3. The improved SLM of claim 1, wherein the SLM is a LCD and the pixel array is an array of liquid crystal elements.
 4. The improved SLM of claim 1, wherein the diode is an n−well diode.
 5. The improved SLM of claim 1, wherein the diode is reverse biased.
 6. The improved SLM of claim 1, wherein the diode is biased in electrical connection with an operating voltage of the SLM.
 7. An improved spatial light modulator (SLM) having an array of pixel elements and control circuitry, the improvement comprising: at least one diode fabricated within the pixel array, the diode operable to conduct current in response to light incident on the pixel array; and a disable circuit operable to receive the current from the diode and to deliver a disable signal to the control circuitry when the current exceeds a predetermined amplitude.
 8. The improved SLM of claim 7, wherein the SLM is a DMD and the pixel array is an array of micro-mirror elements.
 9. The improved SLM of claim 7, wherein the SLM is a LCD and the pixel array is an array of liquid crystal elements.
 10. The improved SLM of claim 7, wherein the diode is an n−well diode.
 11. The improved SLM of claim 7, wherein the diode is reverse biased.
 12. The improved SLM of claim 7, wherein the diode is biased in electrical connection with an operating voltage of the SLM.
 13. A method of detecting light incident on a spatial light modulator (SLM), the SLM having an array of pixel elements and control circuitry, the improvement comprising: placing a diode proximate to pixels of the pixel array, the diode operable to conduct current in response to light incident on the pixel array; and providing electrical connection between current through the diode and a sensing circuit; and detecting the current through the diode using the sensing circuit.
 14. The method of claim 13, further providing the step of determining whether the current exceeds a predetermined amplitude.
 15. The method of claim 14, further comprising the step of providing an electrical connection from the sensing circuit to a control circuit of the SLM, and of disabling the SLM when the current exceeds a predetermined threshold.
 16. The method of claim 13, wherein the placing step is performed by providing a field diode around at least a portion of the perimeter of the array.
 17. The method of claim 13, wherein the placing step is performed by providing at least one diode within the array.
 18. The method of claim 13, wherein the SLM and the diode are fabricated using semiconductor fabrication techniques.
 19. The method of claim 13, wherein the SLM is a DMD and the pixel array is an array of micro-mirror elements.
 20. The method of claim 13, wherein the SLM is a LCD and the pixel array is an array of liquid crystal elements.
 21. An array of digital micromirror pixel elements, comprising: a mirror layer having a reflective mirror associated with each pixel element; a hinge layer spaced under the mirror layer, the hinge layer having a torsion hinge under each mirror and attached to the mirror such that the mirror may tilt above the hinge layer; an address layer spaced under the hinge layer, the address layer having circuitry for controlling operation of the pixel elements; a diode placed around at least a portion of the perimeter of the pixel elements, the diode operable to conduct current in response to light incident on the mirrors; and a disable circuit operable to receive the current from the diode and to disable operation of the pixel elements when the current exceeds a predetermined amplitude.
 22. An array of digital micromirror pixel elements, comprising: a mirror layer having a reflective mirror associated with each pixel element; a hinge layer spaced under the mirror layer, the hinge layer having a torsion hinge under each mirror and attached to the mirror such that the mirror may tilt above the hinge layer; an address layer spaced under the hinge layer, the address layer having circuitry for controlling operation of the pixel elements; at least one diode fabricated within the mirror layer, the diode operable to conduct current in response to light incident on the mirrors; and a disable circuit operable to receive the current from the diode and to disable operation of the pixel elements when the current exceeds a predetermined amplitude. 